1. Field of the Invention
The present invention relates to a phosphor screen and a method of manufacturing the same and an X-ray image intensifier using the phosphor screen and, more particularly, to improvements in an output phosphor screen of an X-ray image intensifier.
2. Description of the Related Art
An X-ray image intensifier generally has a vacuum envelope, an input screen provided at an input side of the vacuum envelope, an output screen provided at its output side, and a focusing electrode provided along the side wall of the vacuum envelope. The output screen is constituted as shown in FIG. 1. That is, a phosphor layer 32 is formed on a face plate 31 consisting of a glass having polished upper and lower surfaces, and an aluminum layer 33 is formed as a metal back on the phosphor layer 32.
The phosphor layer 32 may be formed by a slurry coating method or an electrodeposition method by using a phosphor powder having a grain size of 1 to 3 .mu.m, or by a deposition method by using such phosphor or a phosphor component. The former method is generally superior in luminance, and the latter method is generally superior in resolution.
Functions of the aluminum layer 33 are to reflect light emitted from a phosphor substance toward an electron beam source to increase a luminance on a phosphor screen, prevent charging of an output phosphor film 12 by an electron beam, and protect the output phosphor film 12 from an alkali atmosphere in a cathode ray tube. If the aluminum layer 33 is formed directly on the phosphor layer, a satisfactorily smooth aluminum layer cannot be easily obtained. For this reason, a nitrocellulose film is normally formed on a phosphor film first, and then an aluminum layer is formed thereon. Thereafter, a heat-treatment is performed to evaporate and remove nitrocellulose from gaps between aluminum molecules. The resultant aluminum layer is formed on the phosphor layer. The aluminum layer formed in this manner has good smoothness.
In such a conventional output phosphor screen, however, a part of light emitted from the phosphor layer is not output from the output phosphor screen but scattered in a lateral direction in the phosphor layer. This scattering or irregular reflection is a main cause of reduction in contrast and resolution of the output phosphor screen. This scattering phenomenon is shown in a schematic view of FIG. 2. As a shown in FIG. 2, a phosphor is excited upon radiation of, e.g., an electron beam 35, and light emission occurs at a light-emitting point X of the phosphor. A part 37 of the emitted light is reflected by an interface 36 between a phosphor layer and a substrate and then reflected by an aluminum layer 33. The emitted light is scattered by repetition of this reflection, i.e., multiple reflection.
In order to prevent multiple reflection, the smoothness of a metal back may be reduced. If, however, the smoothness is reduced, the reflectivity of the metal back is reduced accordingly to reduce the luminance of the output phosphor screen. In addition, an effect of protecting the phosphor layer from the alkali atmosphere is also reduced.
Such a multiple reflection phenomenon also occurs on the input phosphor screen. As a means for preventing this phenomenon, Published Unexamined Japanese Patent Application No. 50-109662 discloses a technique for spreading glass beads all over a substrate and forming a phosphor layer on the obtained glass beads layer by deposition. Glass beads, however, have a low heat resistance, and it is difficult to form particles of glass beads. Therefore, this technique cannot be applied to the output screen.
As described above, it is difficult to obtain an output phosphor screen having satisfactory luminance, contrast, and resolution.